This invention relates to an improved method for prilling urea by allowing molten droplets of a substantially water-free urea melt to fall downwardly through a cooling zone, countercurrent to a cooling gas in which a seeding material has been dispersed.
It is known from U.S. Pat. No. 3,450,804, that urea prills can be made by spraying a substantially water-free urea melt downwardly through a cooling zone countercurrent to a cooling gas in which solid urea particles, or particles of some other seeding material, have been dispersed. In this known method, the principal object of which is a substantial reduction of the amount of absorbed moisture in the prilled product, the seeding material is present in the form of very finely divided solid particles, preferably as a fog or in colloidal form. Optimum results are said to be obtained by using finely divided particles in the form of a fog of urea decomposition products produced by contacting solid urea with a red hot metal plate.
It has been found that in applying this known method, a limited number of large crystallites will form in the cooling droplets from the surface inwards, which crystallites are oriented in essentially the same direction. As the droplets cool, the outer skin, which is formed first, is locally sucked inwards as the interior portion of the droplet cools and solidifies, resulting in the formation of cavities such as shown in FIG. 1. As a consequence, the prills thus formed have a low impact strength, and are thus susceptible to pulverization and dust formation during transport and processing of the prills. This disadvantage is particularly apparent during bulk transport of the urea prills, which mode of transport is being increasingly utilized.
It is, therefore, an object of this invention to provide an improved method for prilling urea whereby urea prills of increased impact strength and less susceptibility to crushing are formed.